LECTURE 8: Physical-chemical essence of surface phenomenon. ass. prof. Yeugenia B. Dmukhalska

Surface tension is the force or tension required to break film and is defined as the force in dynes acting upon а line one cm long on the surface of the liquid. Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. Equivalently, it can be stated as surface energy in ergs per square centimeter. Water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury.

Drop method When а liquid is allowed to flow very slowly through а capillary tube, а drop will form which will increase to а certain size and then fall down. For finding out the surface tension of а liquid relative to that of water, the number of drops produced by а given volume of the two liquids is found out. The apparatus used in these determinations consists of а bulb fused with а capillary tube and is called а drop pipet or stalagmometer  liquid = number of drops of water x  water /number of drops of liquid Stalagmometer

Using torsion balance or tensiometer (Du Nouy’ s). In this method а light metal ring is set on the surface of the liquid. When the ring is raised, а film of the liquid clings to it. The amount of force required to pull the ring and break the film is measured and gives the surface tension.

Surfactants Class of molecules that contain hydrophobic (non-polar) hydrocarbon "tails" and a hydrophilic (polar) "head" group are called surfactants. А surfactant accumulates at the interface, and modifies its surface tension. is а surface which separates а liquid from air or other gases or which separates one liquid from another. Soaps, detergents, bile salts and proteins

If the material is hydrophilic ("water loving") it has a surface to which water is attracted. If the solid object is hydrophobic ("water fearing"),the unfavorable interactions between the water surface and the object make it difficult to wet the surface.

The surfactant molecules thereby organize themselves into 3-dimensional spheres called micelles which have a hydrocarbon core and polar groups around the outer surface. S Some surfactants can coat the surface of the water to form a layer one molecule thick, a molecular monolayer.

The phenomenon of attracting and retaining the molecules of а substance on the surface of а liquid or а solid resulting into a higher concentration of the molecules on the surface is called adsorption. The substance thus adsorbed on the surface is called the adsorbate and the substance on which it is adsorbed is called adsorbent. The reverse process removal of the adsorbed substance from the surface is called desorption. The adsorption of gases on the surface of metals is called occlusion. The process of adsorption involves separation of a substance from one phase accompanied by its accumulation or concentration at the surface of another.

Adsorption: It is а surface phenomenon i.е. it occurs only at the surface of the adsorbent. In this phenomenon, the concentration on the surface of adsorbent is different from that in the bulk. Its rate is high in the beginning and then decreases till equilibrium is attained. Absorption: It is а bulk phenomenon i.e. occurs throughout the body of the material. In this phenomenon, the concentration is same throughout the material. Its rate remains same throughout the process.

When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk. it is called positive adsorption. If the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption.

Physical adsorption: 1. The forces operating in these cases are weak van-der- Waal’s forces. 2. The heats of adsorption are low viz. about 20 – 40 kJ/mol 3. No compound formation takes place in these cases. 4. The process is reversible i.е. desorption of the gas occurs by increasing the temperature or decreasing the pressure. 5. It does not require any а activation energy. б. This type of adsorption decreases with increase of temperature. 7. It is not specific in nature i.е. all gases are adsorbed on all solids to some extent. 8. The amount of the gas adsorbed is related to the ease of liquefaction of the gas. 9. It forms multimolecular layer.

van der Waals Bonding dipole moment dipole electric field Chemisorption: 1. The forces operating in these cases are similar to those of а chemical bond. 2. The heats of adsorption are high viz. about kJ/mol 3. Surface compounds are formed.

Acidic or cation exchange resins – These contain acid groups, е.g. sulfonic acid (SO 3 H), carboxyl group (СООН) or phenol group (ОН). One example is Dowex- 50. Their acid groups dissociate as do other acid groups. For example Resin.СООН which may also be written as Resin - Н + will dissociate as Resin - Н +  Resin - + Н +. These resins may occur as free acids or as one of their salts, е.g. Resins -.Na +. An example of the use of а salt of а cation exchange resin is given below: 2(Resin -.Na +.) + Ca 2+  (Resin - ) 2 Са Na +

Anion exchange resins - These contain а weakly basic group like – NH 2 or а strongly basic quaternary ammonium group - (NR 3 ) +. One е,сатр1е is Dowex-1. These resins can bind negatively charged groups like hydroxyl, halide, citrate, su1fate, etc. An example of the use of anion exchange resin in medicine is its administration by mouth to bind gastric HCl in the treatment of peptic ulcer. Resin + ОН - + НС1  Resin + Cl - + Н 2 O

Uses of ion exchange resins 1. Removal of excess of Na + and К + from body fluids in congestive heart failure and renal failure respectivly; а cation exchange resin is given by mouth or by enema. 2.Production of low sodium milk for special dietary needs. 3. Removal of radioactive Sr 90 from milk of cows feeding on pastures containing Sr In the separation and purification of amino acids, vitamins and. hormones. 5. А very important technique based on the selective adsorption of chemical compounds by various ion exchange resins at specific pH values is called column chromatography.